Every fall, you need a new flu shot. That's because today's vaccines train your immune system to recognize specific strains of flu—identified by two proteins on the virus's coat: hemagglutinin and neuraminidase. That's where the 'H' and 'N' come from in H1N1.

Problem is, those proteins are a moving target—they mutate quickly. Once they do, your immune system can't recognize them. And you've got something like the 2009 swine flu, a strain the flu shot never primed us to fight.

To make a more universal vaccine that would work year after year, researchers focused on a smaller, more stable protein called M2. In human strains, the protein has hardly changed since the 1930s.